The invention refers to a method and an apparatus for transmitting an optical orthogonal frequency division multiplex signal.
Orthogonal frequency division multiplexing is a promising modulation technique, which enables to overcome chromatic dispersion in optical transmission systems. OFDM is well known from wireless and wired communications. A plurality of binary information symbols, interpreted as weights of consecutive orthogonal subcarriers, called channels here, are translated from the frequency domain into the time domain via an Inverse Fast Fourier Transform (IFFT) and transmitted via an optical fiber as a block. The next combination of binary information symbols is transmitted in the next block. At the receiver these blocks are translated back to the frequency domain using a Fast Fourier Transform (FFT) and then demodulated.
Chromatic dispersion of optical fibers is a problem for most optical transmission systems. Chromatic dispersion results in different time delays for different frequencies. To overcome chromatic dispersion, it is important to avoid the generation of two side bands, when modulating the OFDM signal onto an optical carrier. Therefore, single-sideband (SSB) modulation has to be applied. The theory of single-sideband modulation for audio signals was described by E. Kettel, Telefunken-Zeitung, Jg. 37 Heft 3/4, especially pages 247-251. The big advantage of “Compatible” SSB-modulation is the highly spectrally efficient signal capable for direct detection. To receive OFDM signals, in general a coherent receiver is required. However, special signal designs also allow the much simpler and thus cheaper direct detection.
One possibility for direct detection, as suggested by Lowery et al, Proceedings of Optical Fiber Communications, OFC 2006, PDP39, pp. 247-259, is to have a gap between carrier and signal with the same width as the OFDM signal itself (see FIG. 2 or inset FIG. 3). This method is named “Offset SSB”. After the photodiode, according to the squaring of the received optical OFDM signal while converting it into an electrical signal, the signal-signal beat product falls into this region, whereas the useful signal (the signal-carrier beat terms) is found adjacent to the gap interval. The drawback of this approach is the large amount of bandwidth occupied by the gap-interval, which implies low spectral efficiency. This method also requires additional modulation onto a radio frequency subcarrier and additional components.